1. Field of the Invention
The present invention is related to the field of blanking either raster or randomly scanned displays and more particularly to an apparatus that turns off the electron beam on a Cathode Ray Tube (CRT) independently of the graphics generator whenever the beam is in a predefined area on the screen.
2. Description of the Prior Art
The prior art contains various types of displays. These various types of displays utilize character generators which are either "stroke generators" or "dot matrix display generators". In a dot matrix type display, characters are formed by sequentially generating a series of discreet spots at predetermined coordinate locations to thereby form an image of the character. In a stroke generator or randomly scanned type display, an alpha-numeric character is presented by generating incremental line segments which when connected together create a visual presentation of a desired character. The present invention is applicable to either type of display, however, the primary use of this invention will be on a stroke generator type display.
In a typical CRT raster scanned visual display system, an image is generated by allowing a scanning electron beam to strike the phosphor on the face of the cathode ray tube only at selected locations and by blanking or keeping off the electron beam as it scans over all other locations. To maintain an image generated in this manner on the cathode ray tube, it is necessary for the electron beam continuously to repeat the same data. To allow this repeated scanning, the data must be stored in a memory. In order to be able to address any point on the face of the cathode ray tube, the memory must contain M storage positions, where M is equal to the number of scan lines per frame, times the number of elements of resolution in one scan line.
In CRT raster scanning visual display systems, the information to be displayed typically is provided by a microprocessor. This information is normally supplied to a CRT controller circuit which generates the horizontal and vertical display sync pulses as well as a display timing signal that determines the horizontal and vertical video blanking times which correspond to the occurrence of the horizontal and vertical sync pulses. In addition, the typical visual display system also utilizes a refresh memory circuit wherein information concerning the visual display to be provided in a single display frame is received from a microprocessor and stored for later recall by the operation of the CRT controller circuit. Also, such systems utilize a character generator which receives control signals from the CRT controller circuit and display information signals from the refresh memory circuit and in response thereto provides excitation signals to a video control circuit that determines the video excitation to be provided on the CRT screen. This video excitation comprises the timed activation of a scanning CRT electron beam gun to produce a desired display character at a desired location on the CRT screen.
U.S Pat. No. 4,663,618 by Hayles discloses an arbitrary raster blanking circuit. In this invention, the horizontal or X-axis and vertical or Y-axis deflection voltages of the raster signal are used to determine the beam position on the screen and then by using voltage comparators, it is determined if the beam is in a predefined area and if it is to blank the beam. By using voltage dividers, operational amplifiers (op amps), potentiometers and comparators it is possible to define and adjust the circuit for arbitrary shapes. Each different shape that is to be blanked would require it's own set of voltage dividers, op amps, potentiometers and comparators.